This invention relates to the external alignment lights of a system for performing tomography. External alignment lights indicate the proper staged position of a patient prior to translating the patient to a scanning position for performing tomography.
Tomography is a technique for constructing a cross-sectional image of a patient. The anatomy to be studied is placed between a source of a fan shaped beam of penetrating radiation (typically X-rays) and a detector for such radiation. The source and detector are synchronously orbited about the patient, keeping the anatomy of interest in the plane of the radiation. Using the analog signals from the detector, the attenuation of the radiation by each point in the section of the patient being scanned can be determined and displayed as a cross-sectional image of the scanned area.
In a typical system for performing tomography, the radiation source and detector are mounted in a gantry for rotation about a generally cylindrical opening. The patient is positioned on a generally horizontal patient support or table. The table is translatable longitudinally into the cylindrical opening to move the patient from a staged position near or outside the opening to a scanning position within the opening. Either the gantry, the table, or both can be tilted to provide oblique scans through the patient.
It is known that to successfully scan the anatomy of interest the patient must be precisely positioned in relation to the scan plane.
One means for ensuring proper alignment of the anatomy of interest has been to project one or more beams of light on the patient to illuminate the anatomy which is then in the scan plane. Such lights, commonly called internal alignment lights, require the patient to be in the scanning position to check alignment. But the scan plane is deep within the gantry, making the position of the patient difficult to observe and adjust using internal alignment lights alone.
Another prior alignment system is shown in U.S. Pat. No. 4,117,337, issued to Staats on Sept. 26, 1978. This patent is hereby incorporated by reference. In this patent, external alignment lights project a pattern on the staged patient illuminating the section of the patient which will be in the scan plane when the patient is translated into the gantry. The longitudinal distance between the section of the staged patient defined by the external alignment lights and the scanning plane is equal to the programmed translation of the patient into the gantry. In the patented system, when the gantry or the patient table is rotated about a lateral tilt axis to provide oblique scans, the side alignment lights must be manually or mechanically tilted if they are to properly identify the scan plane. The ceiling mounted alignment light must be shut off altogether when the gantry or patient table is angulated, since the pattern it projects on the patient does not coincide with the portion of the staged patient to be scanned.
Another prior art alignment system uses an external alignment light mounted to the gantry to project a beam of light in a plane parallel to the scan plane. The pattern cast on the patient by such an external alignment light is subject to parallax when the gantry or the patient table is angulated. The prior art does not show how to compensate for this parallax error.